Yolk sac generally refers to the yolk sac, the diameter of the yolk sac 0.4 cm is generally normal, but due to the different circumstances of individuals, the specific needs to be combined with their own situation, a comprehensive judgment. Yolk sac is an intermediate segment of the embryo in development, when the egg and sperm combine, it can form a blastocyst, when the blastocyst is planted in the uterine cavity, it can form a yolk sac. Patients can directly observe the yolk sac through ultrasound, which can gradually develop into a germ, and if the germ and the heart tube are pulsating, it indicates that the embryo is well developed. Generally the normal range of yolk sac is 0.3~0.8 cm, pregnant women in the yolk sac diameter of 0.4 cm, which is normal. However, if the ultrasound examination, did not find the yolk sac heart tube pulsation, can be considered to be stillbirth or miscarriage due to some patients can be accompanied by abdominal pain, abnormal vaginal bleeding, diarrhea and other uncomfortable symptoms, should be taken seriously. Pregnant women need to take into account their own situation, as well as the results of the examination, and ask the doctor’s guidance. Ensure sufficient rest time in daily life and do not overwork.